AGU Advances最新文献

Observed temperature changes from 2002 to 2022 reveal a pronounced warming of the Southern Hemisphere (SH) subtropical lower stratosphere, and a cooling of the Antarctic lower stratosphere. In contrast, model simulations of 21st-century stratospheric temperature changes show widespread cooling driven by increasing greenhouse gases, with local warming in the Antarctic lower stratosphere due to ozone healing. We provide evidence that these discrepancies between observed and simulated stratospheric temperature changes are linked to a slowdown of the Brewer-Dobson Circulation, particularly in the SH. These changes in the stratospheric circulation are strongest from October through December. This altered circulation warms the SH subtropical lower stratosphere while cooling the Antarctic lower stratosphere, canceling and even reversing the Antarctic ozone recovery that would have occurred in its absence during this period. When circulation changes are accounted for, the SH subtropical lower-stratospheric warming is removed, and Antarctic lower-stratospheric warming is revealed with enhanced ozone healing, highlighting the crucial role of the stratospheric circulation in shaping temperature and ozone changes.

Understanding the nature of planetary bow shocks is beneficial for advancing our knowledge of solar wind interactions with planets and fundamental plasma physics processes. Here, we utilize data from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft to investigate the Martian bow shock, revealing its distinctive characteristics within our solar system. We find that unlike other planetary shocks, the reformation of Mars's bow shock driven by the ultra-low frequency (ULF) waves is more global and less dependent on shock geometries. This distinct behavior is attributed to the broad distribution of ULF waves in the upstream region at Mars, generated not only by shock-reflected ions but also by planetary protons. Additionally, during the reformation process, the amplitude of the ULF waves and the steepened structures are significantly large. This results in the newly reformed shock exceeding the original one, a phenomenon not observed at other planets under similar shock conditions. Therefore, the ULF waves significantly enhance the complexity of shock dynamics and play a more substantial role at Mars compared to other planets.

We report the discovery of a new type of aurora, namely, double auroral undulation that occurred simultaneously on both equatorward and poleward edges of the duskside auroral oval during the main phase of the May 10–11, 2024 superstorm. Whereas the equatorward auroral undulation (EAU) has been frequently observed and well known, the poleward auroral undulation (PAU) has never been observed yet. Both EAU and PAU were found in the N₂ LBHL (165–180 nm, mostly due to electron precipitation) and Lyman α 121.6 nm (due to proton precipitation) images acquired by the Defense Meteorological Satellite Program (DMSP) spacecraft. The spatial amplitude and wavelength of the PAU reached ∼900 km. During the PAU crossing, DMSP observed both precipitating electrons and ions (protons) and a plasma drift up to ∼5,000 m/s with a strong shear. Simultaneous ground-based all-sky imaging at South Pole captured the dynamic behavior of the PAU that propagated in an anti-sunward direction at a phase speed of ∼250 m/s. The solar wind conditions during the PAU were a high solar wind speed (∼700 km/s), an intense southward interplanetary magnetic field (IMF, approximately −40 nT), and a high density (37 cm⁻³). These upstream conditions suggest that the generation of PAU is likely due to giant Kelvin-Helmholtz waves on the magnetopause for southward IMF.

Cities play a crucial role in reducing global greenhouse gas emissions. While activity-based (“bottom up”) emission estimates are widely used for global cities, they often lack independent verification. In this study, we use remotely-sensed CO₂ observations from the Orbiting Carbon Observatory-3 (OCO-3) to “top-down” estimate CO₂ emissions for 54 global cities. This global-scale analysis is enabled by a computationally efficient cross-sectional flux approach, which uses NO₂ observations from TROPOMI and trajectory simulations from HYSPLIT to identify OCO-3 pixels influenced by urban plumes. Our satellite-based emission estimates for 54 global cities agree within 7% to two widely used bottom-up data sets but reveal regional discrepancies. Bottom-up estimates tend to overestimate emissions for cities in Central East Asia and South and West Asia, while underestimating emissions in Africa, East and Southeast Asia & Oceania, Europe, and North America. Additionally, our satellite-based socioeconomic analysis shows that (a) high-income cities tend to have less carbon-intensive economies: North American cities emit 0.1 kg CO₂ per USD of economic output, while African cities emit 0.5 kg CO₂ per USD, and (b) per capita emissions decrease with increasing population size, from 7.7 tCO₂/person for cities under 5 million residents to 1.8 tCO₂/person for cities over 20 million residents. This study highlights the potential of satellite data to bridge gaps between top-down and bottom-up emission estimates, enhancing the robustness and transparency of emissions monitoring. Our findings emphasize the growing role of satellite data in verifying urban CO₂ emissions and supporting efforts to mitigate emissions for global cities.

Auroral Kilometric Radiation (AKR), the dominant radio emission from Earth, has been extensively studied, though previous analyses were constrained by limited spacecraft coverage. This study utilizes long-term observations from Polar, Wind, and Arase spacecraft to generate comprehensive global AKR occurrence rate maps, revealing a high-latitude and nightside preference. A detailed investigation of the equatorial shadow region confirms that the dense plasmasphere blocks AKR emissions across all wave frequencies. Low-frequency emissions (<100 kHz) are presents outside the shadow region at larger radial distance, which is attributed to magnetosheath reflection, while higher-frequency emissions (>100 kHz) propagate via plasmaspheric ducting and leakage, filling the equatorial region immediately outside the plasmasphere. Ray-tracing simulations identify low-density ducts within the plasmasphere as crucial channels that enable AKR to penetrate the dense plasmasphere, particularly at higher frequencies. These results align with meridional AKR observations, offering new insights into AKR propagation patterns.

Simulating the Earth system is crucial for studying Earth's climate and how it changes. Modeling approaches that simplify the Earth system while retaining key characteristics are important tools to advance understanding. The simplicity and flexibility of idealized models enables imaginative science and makes them powerful educational tools. Evolving scientific community needs and increasing model complexity, however, makes it challenging to maintain and support idealized configurations in cutting-edge Earth system modeling frameworks. We call on the scientific community to re-emphasize model hierarchies within these frameworks to aid in understanding the Earth system, advancing model development, and developing the future workforce.

Observational analyses consistently find that yields of major rainfed crops increase with temperature up to a threshold of approximately 32$��°�$C, above which they reduce sharply. Two damage pathways have been suggested to explain this relationship: that high temperatures directly stress crops and drive yield loss, or that high temperatures induce water stress in crops, which in turn drives yield loss. Here we explore a third pathway: that soil water stress limits both agricultural productivity and evaporative cooling, giving rise to the nonlinear relationship between temperature and yield. Determining which of these pathways underpins the yield-temperature relationship is important for predicting future crop productivity because climate change is expected to alter the co-variability between temperature and water availability. To examine this third pathway, we use cumulative growing-season transpiration from an idealized land surface model as a proxy for yield. This approach reproduces the observed yield-temperature relationship, even though the model includes no mechanisms that limit productivity at high temperatures. In experiments where the influence of temperature on soil moisture is suppressed, yields still decline during hot, dry periods in a manner consistent with the observations. We conclude that water stress, and its influence on evaporative cooling, temperature, and agricultural productivity, drives the yield-temperature relationship found in crops that experience episodic water stress. This framework explains the muted sensitivity of irrigated yields to high atmospheric temperatures, and suggests that future yield outcomes depend more critically on changes in rainfall than suggested by estimates that attribute yield losses primarily to temperature variations.

Winter waves in the polar jet stream are associated with extreme cold outbreaks and can modulate longer-term winter temperature trends in the mid-latitudes. Recent research has highlighted a positive trend in jet stream waviness from 1990 to 2010, with a hypothesized connection to Arctic amplification of anthropogenic warming. However, an increase in jet stream waviness has also been hypothesized to contribute to the winter “warming hole” (WH) in eastern North America, a cooling phenomenon from 1958–1988, beginning several decades prior to the recent waviness trend. These potentially conflicting hypotheses highlight the uncertainty of long-term jet stream waviness variability prior to the satellite era (1979–present). Here we develop a new record of wintertime jet stream waviness spanning 1901–2023 based on self-organizing maps and nine different temperature and reanalysis data sets with the dual purpose of (a) understanding the historical variability of polar jet stream waviness in the eastern United States, and (b) quantifying the impact of jet stream waviness on WH-era surface temperatures. Our analysis reveals elevated jet stream waviness in the 1960s–1980s that surpassed modern waviness levels, and we find that jet stream waviness contributed to two-thirds of winter WH cooling beginning in 1958. These results are consistent with a strong connection between temperature trends in the eastern U.S. and jet stream troughing but indicate that additional mechanisms also contributed to the WH. Our analysis further highlights that recent increases in jet stream waviness are well within the range of early to mid-20th century variability, prior to the emergence of Arctic amplification.

Wildfires in the western US increasingly threaten infrastructure, air quality, and public health. Prescribed (“Rx”) fire is often proposed to mitigate future wildfires, but treatments remain limited, and few studies quantify their effectiveness on recent major wildfires. We investigate the effects of Rx fire treatments on subsequent burn severity across western US ecoregions and particulate matter (PM_2.5) emissions in California. Using high-resolution (30-m) satellite imagery, land management records, and fire emissions data, we employ a quasi-experimental design to compare Rx fire-treated areas with adjacent untreated areas to estimate the impacts of recent Rx fires (Fall 2018–Spring 2020) on the extreme 2020 wildfire season. We find that within 2020 wildfire burn areas where Rx fires were used prior to 2020, burn severity changed by −16% (p < 0.001) and smoke PM_2.5 emissions changed by −101 kg per acre (p < 0.1). Rx fires in the wildland-urban interface (“WUI”) were less effective in reducing burn severity and smoke PM_2.5 emissions than those outside the WUI. Overall, Rx fires led to a net reduction of −14% in PM_2.5 emissions, including those from the Rx fires themselves. The proposed policy of treating one million acres annually in California could reduce smoke emissions by 655,000 tons over the next 5 years, equivalent to 52% of the emissions from 2020 wildfires. Our analysis provides comprehensive estimates of the net benefits of Rx fire on subsequent burn severity and smoke PM_2.5 emissions in the western US, an empirical basis for evaluating proposed Rx fire expansions, and valuable constraints for future modeling.

Past research has shown that multiple climate subsystems might undergo abrupt shifts, such as the Arctic Winter sea ice or the Amazon rainforest, but there are large uncertainties regarding their timing and spatial extent. In this study we investigated when and where abrupt shifts occur in the latest generation of earth system models (CMIP6) under a scenario of 1% annual increase in CO₂. We considered 82 ocean, atmosphere, and land variables across 57 models. We used a Canny edge detection method to identify abrupt shifts occurring on yearly to decadal timescales, and performed a connected component analysis to quantify the spatial extent of these shifts. The systems analyzed include the North Atlantic subpolar gyre, Tibetan Plateau, land permafrost, Amazon rainforest, Antarctic sea ice, monsoon systems, Arctic summer sea ice, Arctic winter sea ice, and Barents sea ice. Except for the monsoon systems, we found abrupt shifts in all of these across multiple models. Despite large inter-model variations, higher levels of global warming consistently increase the risk of abrupt shifts in CMIP6 models. At a global warming of 1.5°C, six out of 10 studied climate subsystems already show large-scale abrupt shifts across multiple models.